Thermal cracking of hydrocarbon feeds in the presence of steam (“steam cracking”) is a commercially important technology for producing light olefins such as ethylene, propylene, and butadiene. Hydrocarbon feeds can include one or more of ethane, propane, butane, naphtha, heavy gas oils, and crude oil. Steam cracking furnaces for carrying out steam cracking generally include a convection section, a radiant section located downstream of the convection section, and a quenching stage located downstream of the radiant section with respect to the flow of hydrocarbon feeds. At least one burner is included in the steam cracking furnace for providing heat to the convection and radiant sections. The burner(s) are located in at least one firebox, the firebox being proximate to the radiant section, with the convection section being located downstream of the radiant section with respect to the flow of heated combustion gases (“flue gas”) produced by the burner. Tubular coils (“tubes”) are utilized for conveying the hydrocarbon feed, steam, and mixtures thereof through the furnace's convection and radiant sections where the hydrocarbon is cracked to produce a steam cracked effluent. The steam cracked effluent is quenched and conducted away for further processing to produce the light olefins.
The hot flue gas generated by the burners is conducted away from the firebox through the convection section and discharged to atmosphere via a flue gas stack. A negative pressure (also called “draft”) is generated in the flue gas stack to facilitate proper combustion and flue gas removal. Additionally, the draft is maintained for safety reasons to prevent hot flue gas from exiting through any openings in the firebox, such as, at an open visual inspection port. Maintaining draft ensures ambient air flows in through any firebox openings instead of hot flue gas exiting the firebox anywhere other than out the stack.
Some steam cracking furnaces are natural draft meaning they rely on the height of the stack and differences between hot flue gas density and density of the cooler ambient air to generate draft. Natural draft furnaces are not preferred in steam cracking because very tall stacks must be designed to provide sufficient draft to meet the desired firing rate or rate of heat generation. Many modern steam cracking furnaces are designed to use a fan or blower as the means of generating draft and have only a very short stack located at the discharge of the blower. In these cases, if the blower ceases operation, either due to mechanical issues or loss of the driving power source, the furnace will lose draft. The flue gas pressure inside the furnace will rise to above ambient pressure. It is normal practice to have pressure sensors located on the furnace which identify when the pressure rises above ambient pressure, and an automatic trip or safety-interlock system that will shut off the fuel supply to the furnace in such a situation. If the full fuel supply is shut off instantaneously, the furnace temperatures will cool down very rapidly, including the temperatures of the radiant tubes. This rapid cool down, by itself, is detrimental to service life of the tubes as contraction and expansion lead to metal fatigue. However, the metal fatigue is exacerbated by the presence of coke formed inside tube walls.
During normal cracking operation the process forms coke on the inside of the radiant tubes. Coke has a coefficient of thermal expansion that is about an order of magnitude lower than that of the radiant section tube material. During a blower shut-off event (e.g., blower failure), the radiant tubes may experience a large, sometimes rapid reduction in temperature. The differential in the coefficient of thermal expansion between the tube metal and the coke layer causes significant physical/mechanical stresses in both materials. If the coke layer is thin, the tube contracts more than the coke crushing the coke within. However, if the coke is thick and the tube metal weakened, the tubes may split and fracture around the coke. Either way, these mechanical and physical stresses lead to tube degradation known as thermal shock. Thus, thermal shock significantly reduces tube lifetime, requiring process down-time, and expenditure of significant capital costs in furnace repair. This issue is most important in ethane cracking, as the coke formed in ethane cracking is the hardest coke formed in steam cracking.
Therefore, a furnace design and process of operating a furnace that reduces or prevents thermal shock is desirable.